Created a custom world with dungeon spawner level 100 (not just 7), found an area with 3 zombie spawners next to each other, and zombies spawned inside solid blocks (some suffocated before moving out). Torches did not seem to effect areas near solid blocks. Needed to dig out all blocks within 8 blocks of spawner to have any effect. Otherwise, zombies continued to spawn within cobble and stone, and even 1 block below the spawner, as well as within 8 blocks, regardless if air or stone.